Process of lowering the viscosity of hydrolyzed cellulose esters



Aug. 9, 1938. Q J MALM 2,126,489

PROCESS OF LOWERING THE VISCOSITY OF HYDROLYZED CELLULOSE ESTERS FiledMay 7, 1956 THE EFFECT OF WATER ON THE VISCOJITYOF ACETONf SOLUTIONS OFORDINARY AND ASH-Hm CflLUlOJE AC1. M7715 SECONDS ACfTO/Vt' macs/"(4:1)r0:

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2 WATER o 2 4 6 a m CONCENTRATION or ACETONE Carl J. Malm ATTORNEYSPatented Aug. 9, 1938 UNITED STATES PATENT OFFICE PROCESS OF LOWERINGTHE VISCOSITY OF HYDROLYZED CELLULOSE ESTERS Application May 7, 1936,Serial No. 78,448

13 Claims.

The present invention relates to the lowering of the viscosity ofhydrolyzed cellulose esters by removing the ash content therefrom. Thisis accomplished by washing with salt-free water, such as distilledwater.

It is commonly accepted in the cellulose acetate art that high viscosityesters, when coated out from their solution in acetone, result in filmor sheeting having, as a rule, good flexibility and high tensilestrength. The viscosity of the ester is regarded as indicating thecomplexity of the cellulose micell and, therefore, the high viscosityesters are less degraded than those of low viscosity, hence the highstrength of products from high viscosity esters. The high viscosity ofesters of undegraded cellulose precludes the use of high concentrationsin organic solvents such as acetone and consequently in some instances,such as in lacquers, it is necessary that lower viscosity esters beemployed to obtain the desired concen-.

tration even though the cellulose has been broken down to some extent.

One object of the present invention is to lower the viscosity ofhydrolyzed cellulose esters without breaking down the cellulose itself,thus making possible high concentrations of esters of undegradedcellulose in organicsolvents. which have not been possible up to thepresent time. Another object of the invention is not only to reduce theviscosity of the hydrolyzed esters but also to substantially remove theash content therefrom which has a decided effect on the viscosity andalso some effect upon the clarity of the product. depending on theamount and kind of mineral salts present. A still further object of myinvention is to provide a cellulose ester having a low viscosity inacetone considering the 'cuprammonium viscosity of the cellulose. Astill further object of my invention is to provide a yarn having a greatdensity and hence greater strength for a given denier than that of theyarn ordinarily prepared.

Contrary to the popular conception that the viscosity of a celluloseester is almost entirely determined by the breakdown of the cellulosemicell, I have found that the viscosity of hydrolyzed cellulose estersmay be measurably diminished by removing'a substantial proportion of theash content therefrom, by a thorough washing with salt-free water,preferably while hot. I have found that this reduction in viscositytakes place without any breakdown of the cellulose as evidenced bythezfact that the cuprammonium viscosity of the regenerated cellulose isthe same as that of the cellulose regenerated from the ester before thereduction in viscosity occurs. I have found that the resulting productrequires only a relatively small amount of solvent to form a flowablesolution which may be employed for the manufacture of film and filamentsand that in the manufacture of plastics a good flow is obtained withcomparatively small amounts of plasticizer. I have found that thetensile strength and flexibility of the film or filaments, prepared fromthese lowered viscosity esters, are as high as the strength andflexibility of products from high viscosity cellulose esters.

About the only practical way of obtaining saltfree water is bydistillation. This distillation may be accomplished naturally, thesalt-free water being obtained in the form of rain or snow, orartificially. It is desirable that great care be used that the distilledwater or the rain water he collected under conditions which avoid anycontact with mineral salts having an alkaline earth metal base.

The viscosity of cellulose acetate or other fatty acid esters ofcellulose is lowered in accordance with the present invention bytreating the ester with salt-free water, preferably at a hightemperature. To be most effective, a series of washings are required tolower the viscosity of the ester. It is desirable, especially when thewashlogs are carried out at room temperature to lower the pH of thedistilled water. This can for instance be done by bubbling carbondioxide through the wash water and this carbon dioxide may be introducedinto the bath of cellulose ester in distilled water through small Jetslocated in the washing tank.

This lowering of viscosity can also be accomplished by adding a small,amount of acid such as formic, acetic, oxalic etc. or inorganic acidssuch as hydrochloric or sulfuric acid to the wash water. It is importantto remove the acid by thorough washing with distilled water before thecellulose ester is dried.

The present invention is concerned primarily with the treatment ofhydrolyzed cellulose esters, as there is little if any effect uponcellulose esters which are fully esterlfied. It is believed that thehydroxyl groups of the cellulose ester loosely hold the mineral saltsand that this loose combination causes the high viscosity of thecellulose ester. Upon washing with hot distilled water, this looselinkage is destroyed and the mineral salts may then be washed out. This,of course, is an explanation based upon the fact that only thehydrolyzed esters are lowered in viscosity by this treatment. It is tobe understood, however, that the applicant is not to be limited by histheory as to the lowering of the viscosity by removal of salts inaccordance with the present invention.

The amount of reduction in the acetone viscosity of a cellulose ester inaccordance with the present invention depends on at least two factorsnamely:

1. The degree of hydrolysis, and

2 The percentage of alkaline earth metal salts which were removed fromthe ester.

It may be seen from the second factor that the cellulose esters having arelatively high content of mineral salts are much more susceptible to myviscosity lowering process than those having a lower mineral saltcontent.

I have found that organic acid esters of cellulose prepared from refinedwood pulp as the starting material are particularly susceptible to thelowering of viscosity by my invention. In all of the examples given,esters from refined wood pulp are employed. It may be seen from theseexamples that the viscosity is reduced considerably, such as more than50% in most in stances and at least 25% in almost every case.

Under the same conditions, namely a series of at least 4 washes withsalt-free water, the acetone viscosity of a hydrolyzed cellulose estersuch as cellulose acetate which has been hydrolyzed to acetonesolubility, which has been prepared from cotton, is reduced at least 25%ordinarily. 0bviously esters prepared from other types of cellulosematerial will respond to removal of the ash content with a lowering ofthe acetone viscosity of the same order. In almost every case areduction of at least 25% and generally considerably more isaccomplished by removal of the mineral salts in accordance with myinvention.

That the mineral salts have been removed is very diflicult to determineby analysis due to the very small amount of that material which ispresent before treatment. That it is the removal of the mineralsaltswhich causes the reduction in viscosity is concluded from the fact thatwhen the ester whose viscosity has been reduced is treated with a verydilute aqueous solution of a mineral salt having an alkaline earth metalbase. the acetone viscosity is increased to its former viscosity or evenbeyond.

The following examples illustrate the preparation of a lowered viscositycellulose ester by means of the present invention:

An ordinary commercial grade cellulose acetate, having a viscosity of 40seconds when dissolved in 4 parts of acetone and an acetyl content of40.5%, was washed 6 times with boiling distilled water and then dried.The acetyl content of the resulting product was still 40.5%, but theviscosity, when the acetate was dissolved in 4 parts of acetone, wasonly 10 seconds.

As another example, some of the 40 seconds viscosity acetate was washedusing 20 changes of distilled water at room temperature. The acetoneviscosity in that case was lowered from 40 to 18 seconds. The 2%cuprammonium visoosity of the cellulose, regenerated from samples beforeand after the wash treatment, gave in both cases 13 centipoises showingthat the Viaeosity of the ester was reduced without in any way affectingthe cellulose itself. In two other cases samples of high viscositycellulose esters were given six washes with hot distilled water. In onecase a cellulose acetate, having a viscosity of 1900 seconds, wasreduced in viscosity to seconds and with the second ester, a viscosityofl45wasreducedto40secondsbythistreatment. The degree of drop inviscosity depends upon the amount and nature of the salts present, thenumber of washes, and the temperature of the distilled water. I havefound that the alkaline earth metal salts, such as of calcium, areespecially efi'ective in maintaining a high viscosity and where suchsalts predominate in the ester before the washing with distilled water,a much greater reduction in viscosity is possible. As a general rule,six changes of hot distilled water is sufficient to obtain the minimumviscosity. It is preferred to use water at a high temperature near tothe boiling point, although the invention is not restricted to the useof any particular temperature of washing. The boiling of the celluloseester in distilledwater is unnecessary and may lower the acyl content,however, it may be employed if a slight reduction of the acyl content isnot objectionable.

Some further instances of lowering the viscosity of cellulose esters inwhich the ester was washed in four 4-hour periods with boiling distilledwater, being rinsed once with distilled water between each boiling gavethe following results:

Viscosity before Viscosity after washing with washing with hot distilledhot distilled water water (in seconds) (in seconds) The viscosity wasdetermined by the falling-ball method in an acetone solution of theester consisting of 4 parts of acetone to one of ester.

As pointed out above, the present invention is directed to the loweringof the viscosity of hydrolyzed fatty acid esters of cellulose whichincludes hydrolyzed cellulose acetate but also other hydrolyzedcellulose esters, such as hydrolyzed cellulose acetate propionate oracetate butyrate or other hydrolyzed simple esters such as cellulosepropionate or cellulose butyrate. As a matter of fact, I have found thatthe more free bydroxyl groups which are present in an ester, the morethe viscosity of the ester is increased by the presence of mineralsalts, or in other words, the more the viscosity may be lowered byremoving the ash content therefrom.

The cellulose esters of lowered viscosity of the present invention arealso eminently suited for the making of other commercial products. whendissolved in acetone. the solution of one of these esters is much moreflowable and consequently more easily handled than a solution of thecorresponding concentration of the high viscosity ester. In spite,however, of this lowered viscosity of solution, the sheeting formedtherefrom has as good a strength and flexibility as that of the highviscosity ester from which it originated The thoroughly washed celluloseesters of lowered vlscosityof my invention are also adapted to thepreparation of yarn. By employing these esters to form synthetic yarn, aproduct is obtained in which the tensile strength is increased 5-10%without any change in the amount of stretch. Also, the othercharacteristics of synthetic yarn, such as aflinity for dyes orpigmenting materials are not affected, and the filaments forming theyarn have a more desirable cross section.

The accompanying drawing illustrates the marked advantage of using anash-free cellulose acetate over using the ordinary commercial product.This drawing shows graphically the relation of the viscosity to theprecentage of water in the acetone solution of two acetone-solublecellulose acetates, the ash-free ester having been prepared from acommercial acetone-soluble cellulose acetate of approximately 40 secondsviscosity upon which the upper curve is based. The upper curve showsthat upon the use of a small amount of water in the acetone employed todissolve the commercial cellulose acetate, the viscosity is much lowerthan with its solution in acetone alone. With the ash-free celluloseacetate, which was prepared from the ester upon which the upper curve isbased, the viscosity in acetone solution is lower than was attained bythe addition of water in the previous case as shown by the lower curveof the drawing. It may also be seen that the mixture of water with theacetone employed to dissolve the ash-free cellulose acetate not onlydoes not lower the viscosity, but if more than 2% water is used, thesolution is more viscous. v

Because of the low viscosity of the acetone solutions of these ash-freeesters, a much greater concentration of the ester in the acetone may beemployed without detrimentally ailecting the handling of the material orthe spinning of the solution. My process also makes possible the use ofesters, the cellulose of which has a higher muprammonium viscosity thanthat which ispossible to employ at present. Therefore, the yarn of myinvention has greater strength than the yarn prepared from celluloseesters, the cellulose of which is more degraded, or in other words. thecellulose of which has a lower cuprammonium viscosity.

The method of making yarn by evaporative spinning is well-known at thepresent time and reference to the prior art,'such as Stone Patent Nos.2,000,047 and 2,000,048 and Clarke and Malm Patent No. 1,969,454 may behad to ascertain satisfactory methods of preparing yarn from acetonesolutions of cellulose esters.

An example of preparing yarn in accordance with my invention is asfollows:

A cellulose acetate prepared from an ordinary commercial grade celluloseacetate, rendered ashfree by washing a number of times with boilingdistilled water and drying was employed. The cellulose acetate of 40.5%acetyl content before washing had a viscosity of 40 seconds whendissolved in 4 parts of acetone. After washing. the acetate had the sameacetyl content but an ace: tone viscosity of only 10 seconds. Celluloseacetate washed repeatedly with distilled water at room temperature torender it ash-free is also satisfactory. In the latter case the acetoneviscosity was lowered from 40 to 18 seconds.

An ash-free cellulose acetate was dissolved in acetone to form a 30%solution of the ester.

This solution was then filtered and passed 7 through a spinneret into anevaporative atmosphere in which warm, dry air circulates. The filamentswere then twisted together to form a thread of unusually high strength.The solution was of higher concentration than can be employedordinarily, the thread was more dense and of greater strength. Althoughthe spinning solution was oi 30% concentration, the solution was easilyflowable and the spinning could be performed at the rate ordinarily usedwith lower concentrations of cellulose esters.

The preferred range of acetyl of cellulose acetate for the preparationof yarn in accordance with my invention is approximately 37-41%. As thisinvention is primarily directed to making possible the preparation offilaments from esters of less degraded cellulose it is applicable, ingeneral, to processes of making yarn from cellulose esters, although itsuse in evaporative spinning processes is especially contemplated.

In the preparation of lacquers, it is desirable that the cellulose esterbe present in fairly high concentration to contribute to the coveringpower of the lacquer. Heretofore'the esters, which have been used forthis purpose, were the esters of a broken down cellulose in order togive a viscosity sufficiently lowered to make the lacquer flowable andyet assure a considerable concentration of the cellulose ester therein.By my invention a lacquer may be prepared from a cellulose ester inwhich the viscosity has been lowered by removing the ash content, and,therefore, the ester present in the lacquer will not have the degree ofbreakdown which, up to the present time, has been thought to benecessary. This results in tougher, stronger and less brittle lacquercoverings.

The lowered viscosity esters of my inventionmay also be employed forvarious other purposes, such as in the preparation of plasticcompositions. Due to the lowered viscosity of these esters, the amountof plasticizer required is less than that ordinarily used, a good flowwill be obtained upon the application of heat and in addition a strongdurable product will be obtained. These properties are especiallydesirable in injection moulding. In the laminating field these estersare also useful either for the preparation of sheeting to be laminatedwith glass, metal or the like, or as a coating solution to be applied tothe surfaces of the sheets which are to be laminated. As the esters ofmy invention have the advantage of the lower viscosity esters in givinga ilowable solution and the advantage of the high viscosity esters inthe undegraded condition of the cellulose, they will, of course, suggestthemselves for many uses.

I claim:

1. The process of lowering the acetone viscosity of a partiallyhydrolyzedacetone-soluble organic acid ester of cellulose by thoroughlywashing the ester with a series or at least four distilled water washes.

2. The process of lowering the acetone viscosity 5 of a partiallyhydrolyzed acetone-soluble cellulose acetate by thoroughly washing theester with a series of at least four distilled water washes.

3. A partially hydrolyzed acetone-soluble lower fatty acid ester ofcellulose, the acetone viscosity of which has been lowered at least by aprocess which comprises thoroughly washing the ester in a series of atleast four distilled water baths.

4. A partially hydrolysed acetone-soluble lower fatty acid ester ofcellulose prepared from refined wood pulp, the acetone viscosity ofwhich .has been lowered at least 25% by a process which comprisesthoroughly washing the ester in a series of at least four distilledwater baths.

5. A partially hydrolysed acetone-soluble cellulose acetate preparedfrom refined wood pulp, the acetone viscosity of which has been loweredat least 25% by a process which comprises thor- 1s series of at leastfour distilled water washes and drying the cellulose ester.

8. The process of lowering the acetone viscosity, by more than 25%, of apartially hydrolyzed acetone-soluble lower fatty acid ester of celluloseprepared from refined wood pulp which comprises thoroughly washingtheester with a series 0! at least four distilled water washes and dryingthe cellulose ester.

9. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble cellulose acetate, by more than 25%, whichcomprises thoroughly washing the ester with a series of at least fourdistilled water washes.

10. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble lower fatty acid ester of cellulose by atleast 25% which comprises thoroughly washing the ester with a series ofat least four distilled water washes, at least one o! the washes beingaccompanied by the introduction of carbon dioxide into the washingliquid.

11. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble lower fatty acid ester of cellulose by atleast 25% which comprises thoroughly washing the ester with a series ofat least four distilled water 'washes, the water in the initial washcontaining a slight amount of acid.

12. A yarn essentially consisting of a partially hydrolyzed lower fattyacid ester of cellulose, the acetone viscosity of which has been reducedby at least 25% by a process which comprises thoroughly washing theester in a series of at least four distilled water baths.

13. A yarn essentially consisting of a partially hydrolyzed celluloseacetate, the acetone viscosity of which has been reduced by at least 25%by a process which comprises thoroughly washing the ester in a series ofat least four distilled water baths.

CARL J. MALM.

CERTIFICATE OF CORRECTION Patent No. 2,126,h89.

August 9, 1958 CARL J. mm.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,first column, line 61;, for F2 1/2" read 2 1/2%; and that the said.Letters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office. I

Signed arfi sealed this 13th day of September, A. D. 1958.

(Soul) Henry Van Arsdale Acting Commissioner of Patents.

series of at least four distilled water washes and drying the celluloseester.

8. The process of lowering the acetone viscosity, by more than 25%, of apartially hydrolyzed acetone-soluble lower fatty acid ester of celluloseprepared from refined wood pulp which comprises thoroughly washingtheester with a series 0! at least four distilled water washes and dryingthe cellulose ester.

9. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble cellulose acetate, by more than 25%, whichcomprises thoroughly washing the ester with a series of at least fourdistilled water washes.

10. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble lower fatty acid ester of cellulose by atleast 25% which comprises thoroughly washing the ester with a series ofat least four distilled water washes, at least one o! the washes beingaccompanied by the introduction of carbon dioxide into the washingliquid.

11. The process of lowering the acetone viscosity of a partiallyhydrolyzed acetone-soluble lower fatty acid ester of cellulose by atleast 25% which comprises thoroughly washing the ester with a series ofat least four distilled water 'washes, the water in the initial washcontaining a slight amount of acid.

12. A yarn essentially consisting of a partially hydrolyzed lower fattyacid ester of cellulose, the acetone viscosity of which has been reducedby at least 25% by a process which comprises thoroughly washing theester in a series of at least four distilled water baths.

13. A yarn essentially consisting of a partially hydrolyzed celluloseacetate, the acetone viscosity of which has been reduced by at least 25%by a process which comprises thoroughly washing the ester in a series ofat least four distilled water baths.

CARL J. MALM.

CERTIFICATE OF CORRECTION Patent No. 2,126,h89.

August 9, 1958 CARL J. mm.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,first column, line 61;, for F2 1/2" read 2 1/2%; and that the said.Letters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office. I

Signed arfi sealed this 13th day of September, A. D. 1958.

(Soul) Henry Van Arsdale Acting Commissioner of Patents.

